Fix alpha threshold GM to test with and without crop rect

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2864

Change-Id: I09f24303b2a693a384fb7399c2287ab476f16319
Reviewed-on: https://skia-review.googlesource.com/2864
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed